Saturday, December 20, 2014

22 December - Chemistry Knowledge History

Vladimir Markovnikov born 1838: synthesis of cyclobutane and cyclopentane derivatives; Markovnikov's rule for additions to alkenes.
William Lloyd Evans born 1870: chemistry of carbohydrates;
John Clarke Slater born 1900: orbital approaches to quantum chemistry (Slater-type orbitals, Slater determinant); tetrahedral carbon compounds.
Arie Jan Haagen-Smit born 1900: nature and source of smog; smog abatement.

Markovnikov's rule for additions to alkenes.

When  the alkene which is unsymmetrical reacts with halogen acid,  two products are possible depending upon the carbon atom to which the halogen atom is attached.

Markovnikov rule: during the addition across unsymmetrical multiple bond, the negative part of the attacking reagent joins with the carbon atom which carries smaller number of hydrogen atoms while the positive part goes to the carbon atom with more hydrogen atoms.

Due to fact that the reaction proceeds according to Markow(v)nikov's explanation, addition of HBr to Propene gives 2-Bromopropene as the major product up to 90%.

Exception to Markovnikov rule - Kharasch effect - Peroxide effect: During the addition of HBr to an unsymmetrical alkene in the presence of organic peroxids (e.g., benzoyl peroxide), Br atom will join to the carbon carrying more hydrogen atoms while H atom will go to the other carbon atom.

Propene + HBR in the presence of Benzoyl peroxide gives 1-Bromopropane.

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